Male and female reproductive organs have different functions and together they form new life. In males, the testes produce sperm. Sperm is released through the penis to fertilize the female.

In females, ovaries produce eggs which travel down through the Fallopian tubes. After intercourse, the male's sperm travels to the Fallopian tubes where they meet up with and fertilize the egg. Fertilization usually happens in the tubes, but can occur in the uterus as well.

When a man ejaculates, he releases between 75 and 900 million sperm. However, it only takes one sperm to fertilize an egg and cause a pregnancy.

Identical twins are formed when one sperm fertilizes one egg and that egg splits into two identical babies. Fraternal twins are formed when two eggs are fertilized by two different sperm. Unlike identical twins, fraternal twins usually look different and may be of two different sexes.

Anthropologists and evolutionary biologists don't know for sure when Homo sapiens discovered the link between sex and pregnancy, but theories suggest that the connection was made between 50,000 and 200,000 years ago.